A glass preform for use in the production of optical fibers comprises a core and a cladding surrounding it. It is required for the core to have a refractive index higher than that of the cladding for the purpose of transmission of light. For example, the glass preform has refractive index difference distribution patterns as shown in FIGS. 1A and 1B. In order to make the refractive index of the core A greater than that of the cladding B, usually, an additive such as GeO.sub.2, Al.sub.2 O.sub.3 and TiO.sub.2 is added to silica glass to increase its refractive index.
The following problems, however, are encountered in adding such the additives:
(1) Increase of the amount of the additive causes light scattering (i.e. Rayleigh scattering). As the amount of the additive added increases, the degree of scattering increases. This is undesirable for light transmission.
(2) Addition of a larger amount of the additive will lead to the formation of bubbles and/or a crystal phase in the preform. GeO.sub.2, for example, yields the bubbles due to generation of GeO gas. Al.sub.2 O.sub.3 is liable to form clusters of Al.sub.2 O.sub.3 crystals. The formation of the bubbles and the crystal phase is undesirable because they increases attenuation.
For these reasons, it is required to minimize the amount of the additive to be added to the core while maintaining the refractive index difference between the core and cladding as high as possible.
For this purpose, it has been proposed to add fluorine to silica glass to lower the refractive index of the preform. In accordance with this method, the additive such as GeO.sub.2 is added to the core to increase the refractive index of the core, thereby achieving a predetermined refractive index difference between the core and the cladding and, thereafter, fluorine is added to lower the refractive index of the whole glass preform while maintaining the refractive index difference between the core and the cladding at the already achieved value, whereupon the apparent amount of the additive added to the core, as determined based on the refractive index of silica (SiO.sub.2), is reduced. In this method, however, there are various problems to be overcome in connection with a procedure of adding fluorine.
Japanese Patent Publication No. 15682/1980, for example, discloses a procedure in which fluorine is added in the course of flame hydrolysis to form glass fine particles. This method has disadvantages such that an absolute amount of fluorine added is small and the required production time is long. This may be due to the fact that moisture contained in the flame reacts with fluorine gas according to, for example, the following reaction formula, thereby yielding HF gas: EQU SF.sub.6 +3H.sub.2 O.fwdarw.SO.sub.3 +6HF (1)
Since HF gas is stable, almost all of fluorine-based gases are converted into HF gas at a high temperature as long as there is moisture. Thus, only a small amount of fluorine-based gases left without being converted into HF gas are used as a raw additive.
Furthermore, HF erodes glass, particularly quartz and easily reacts with silica glass fine particles formed in the flame. This reaction may proceed according to the following reaction formulas (2) and (3). This reaction results in the consumption of the formed glass particles. EQU SiO.sub.2 (s)+2HF(g).fwdarw.SiOF.sub.2 (g)+H.sub.2 O (g) (2) EQU SiO.sub.2 (s)+4HF(g).fwdarw.SiF.sub.4 (g)+2H.sub.2 O(g) (3)
wherein the symbols (s) and (g) represent solid and gas respectively.
Since the fluorine-based gas, therefore, acts to suppress accumulation of the silica glass fine particles, as the amount of the fluorine-based gas added increases, the rate of accumulation of silica glass fine particles decreases, and finally the silica glass particles are not accumulated at all.
Japanese Patent Application (OPI) No. 67533/1980 (the term "OPI" as used herein means a "published unexamined Japanese patent application") discloses a procedure in which a glass soot preform is formed and, thereafter, to the preform is added fluorine by sintering it in an atmosphere containing fluorine. Even in this method, the rate of addition of fluorine is low and, furthermore, the preform is sometimes contaminated with Cu and Fe. Another drawback is that in sintering at a temperature higher than 1400.degree. C., the surface of the glass preform is vigorously etched producing a glass preform having an uneven surface. Still another drawback is that this etching allows easy invasion of impurities contained in a mandrel into the glass soot preform.